Mikhail Gorbachev, media darling, went from piping hot to frosty cold on the last day of his American visit yesterday. He threatened to wear out his welcome with a vengeance.

But first, he exhibited his beguiling salesmanship. At midday, already late for an appointment at the White House, Gorbachev ordered his limousine halted so he could jump out and press flesh at Connecticut Avenue and L Street NW. He supposedly wanted to meet real Americans.

He also, no doubt, wanted to create a scene no self-respecting news camera could refuse. The footage showed up everywhere, and a viewer could hear startled citizens shout questions like "When you coming back?" while Secret Service men barked, "Get your hands out of your pockets."

They were barking at the citizens, not the Communist Party boss, who smiled broadly, or as broadly as he can smile, shook hands and ingratiated himself with the people on the street and the cameras that recorded the scene.

Much marveling over Gorbachev's media smarts has been heard all week, but later yesterday, Gorbachev appeared to unlearn all his lessons and revert to precisely the stereotype he had been so lavishly praised for contradicting. What was billed as a press conference at the new Soviet compound turned into a nearly interminable monologue. Helpless reporters sat strapped into their headsets.

It was just a lot of Gorbledygook.

His charm evaporated. Even fellow bureaucrats on the dais with him looked as though they were nodding off. He seemed not friendly and approachable but testy, even pugnacious. He spoke of his utter disinterest in granting interviews by saying all interviewers ask the same questions and suggesting the questions are too cheeky.

"And what are we going to talk about -- to beat the air?" he said through his interpreter. He was beating the air to death himself. He spoke for more than an hour before taking a single question.

The networks and local stations justifiably bailed out of the Gorbachev appearance long before it ended, but the Cable News Network (CNN) was committed to remaining with it. Regularly scheduled programs began to fall like dominoes as Gorbachev plodded on.

Earlier, on ABC, Sam Donaldson told Peter Jennings that Gorbachev's three days in Washington had been "a great PR blitz," but Gorbachev certainly put a crimp in the blitz yesterday. Perhaps it was a good thing, in a way, that Gorbachev offered such a counterproductive performance. We didn't have to wait for him to leave, and the postsummit analysis to start, for the bubble to burst.

And if it didn't burst, it sure wheezed a little. "New thinking" and "glasnost" and "perestroika" began to seem less like breakthroughs and more like buzzwords from a new Madison Avenue ad campaign.

The Soviets have fitful media smarts. They are not fluent in the new rules of video-age perception management. But they are definitely interested in how they come across on television.

Since the Soviet Embassy is in the District and therefore still unwired for cable TV (and because there are reportedly too many antennas and dishes on the embassy roof for a microwave receiver to be squeezed in), the Russians made a deal with Turner Broadcast Services to receive tapes of all the summit-related programming seen on the Ted Turner-owned CNN.

A CNN spokeswoman confirmed yesterday that Ted Turner Jr., a TBS executive, had been making regular deliveries to the embassy of this week's CNN programming on VHS tapes recorded at the CNN bureau here.

"We are proud of our coverage and would like everyone to see it, including the Russians," the spokeswoman said. She was unable to determine how many hours of coverage were delivered to the embassy but said that 70 hours of summit events were "monitored" by CNN staff members.

Only material that actually appeared on CNN -- no outtakes or unused footage -- was supplied to the Soviets, the spokeswoman said. The senior Turner, a guest at some of the embassy events, has had dealings with the Russians before. Last year, the Soviets collaborated with Turner on the spectacular but little-watched Goodwill Games, an alternative to the Olympics.

CNN has devoted itself exhaustively to the summit, and CNN's Washington anchor, Bernard Shaw, has proven tireless and authoritative on the air. Yesterday, he had to spend a considerable amount of time ad-libbing while waiting for delayed events to transpire.

At one point he went to CNN correspondent Stuart Loory, stationed at the Hay-Adams Hotel. Loory related an anecdote about Charles de Gaulle who, he said, was so vain that he refused to wear glasses in public appearances and, as a result, went about shaking hands with his own security personnel thinking they were the men and women of France.

The picture of Loory broke up as he finished the story, and Shaw could be seen breaking up when the director switched back to him at CNN studios here.

Jennings allowed on the air yesterday that tattlings about strained relations between Raisa Gorbachev and Nancy Reagan "might have been somewhat overreported." It could also be that the charm and conviviality of the Gorbachevs was overestimated. Mrs. Gorbachev seemed rude when she broke away from Mrs. Reagan at the White House so she could prattle on to reporters.

The Reagans appeared to be much better hosts than the Gorbachevs were guests. Saying goodbye under umbrellas that protected them from a Washington downpour yesterday, Ronald and Nancy looked like the proverbial good neighbors, or a particularly benign pair of in-laws. The president smiled warmly and shook Gorbachev's hand.

It could have been any parting of hosts and house guests, except that the two men had spent part of the week tossing around the future of the world, and helping see to it that there might be one.

One half expected Mrs. Reagan to hand Mrs. Gorbachev a jar of homemade peach preserves.

Gorbachev's marathon news conference performance made one worry that perhaps Reagan might have conceded a point or surrendered a bargaining chip merely because he was being outtalked. Apparently no one among the Soviet entourage had the authority to say, "Thank you, Mr. General Secretary," the way it's done at White House press conferences.

Finally, at 7:39, the horrible thing ended, and Shaw, on CNN, breathed a sigh of relief. He said the news conference had clocked in at just under two hours.

"Get out the Guinness Book of World Records," cracked Tom Braden on CNN a few moments later; he said he was sure Gorbachev must have broken one. Asked by Braden if Gorbachev had "spoiled it all" with his "endless" press conference, former national security adviser Zbigniew Brzezinski said, "No, I don't think he spoiled it all, because most people didn't watch it."

Good point, Zbiggy. But those that did had their consciousness braised.

Among the other accolades showered on the Gorbachevs, it was said that both have steady gazes -- or "great eye contact," as Annapolis student Heather Iliff, who met Gorbachev, said when interviewed on Channel 9's "Eyewitness News." They may have great eye contact, but they need to work on camera contact.

By next week it will be hard to remember the euphoria of this week, the excitement of watching the events unfold on TV, the endearing way Mrs. Gorbachev had of saying "Goot morning" to surging throngs of quote-hungry reporters, the baffling way Dan Rather had of referring to Washington as "the mud flats of the Potomac." Perhaps we will have occasion to look back and think we were naive or bamboozled.

But for now, and despite last night's vaudeville routine by the garrulous Gorby, the spell still hangs in the air. Not on it, maybe, but in it.